The present invention pertains to sensors and particularly to fiber optic sensors. More particularly, the invention pertains to optically powered fiber sensors.
Although fiber optic sensors offer great potential, few such products are readily available on the market. The related art reveals four concepts for standardizing and multiplexing fiber optic sensors for a system (e.g., aircraft). These concepts include interferometric phase modulation, time domain modulation, optical spectrum modulation and carrier intensity modulation.
Interferometric phase modulation utilizes interference between signal and reference light beams on single mode fibers. Sagnac interferometers are used for rotation-sensing gyros and optical resonators. Mach-Zehnder interferometer configurations are most often used for applications such as fiber optic underwater hydrophones and magnetic field sensors. Research shows these types of sensors to be quite vulnerable to noise sources such as vibrations, laser instabilities, and temperature drifts. Also it is difficult to interface these sensors to a standard optical interface because of their high instrumentation requirements and specialization.
Sensors which employ the principle of time domain modulation of optical power encode sensed information in optical power level as a function of time. Two types of this modulation are pulse duration and pulse delay. In pulse duration, the length of the optical pulse varies with sone environmental parameter. An example is the fluoresence time length due to an optical pulse impinging on a temperature-dependent fluorescent material in a temperature sensor. In pulse delay, the optical path length varies according to some physical basis causing the variation in the arrival or return time of an optical pulse. Radar and optical time domain reflectometry are two examples. A microbending sensor has been used in the optical time domain reflectometry scheme to implement a pressure sensor/transducer. Such use resulted in inaccurate, inefficient and costly, but needed, instrumentation. Also, such scheme has a high sensitivity to environmental vibration. It is difficult to build small rugged sets of delay lines resistant to microbending optical losses. Multiplexing more than one sensor to a fiber for this scheme is difficult.
In optical spectrum modulation, the spectrum of an optical source is directly modified by some physical process which may include wavelength dependent variations in the refractive index propagation constant, or wavelength selective transmission using some type of chromatic dispersion element. Some problems with this approach of modulation are the decrease in the signal-to-noise ratio from division of the energy into a number of spectral bands, difficulties in multiplexing sensors and developing a standard interface, and the problem of single fiber approaches having optical source light from the interface module mixed with the data where backscatter from connectors tends to swamp the weak direct current (DC) data signal.
In carrier intensity modulation, some effect is utilized to modify the intensity of the optical light, or carrier, used to convey the sensed information. Simple analog intensity modulation schemes are rarely used because of their sensitivity to variations in optical source intensity and fiber interconnect losses. Data encoding schemes, in which the magnitude of the sensed parameter is linearly mapped into a carrier modulation frequency (double sideband and intensity modulation), are popular. Schemes using this voltage to frequency conversion technique to transmit transducer information to any optical interface module suffer from some drawbacks. These drawbacks include high power required for data conversion and transmission, difficulty in multiplexing several sensors and accurately determining the transmitted frequency in a short amount of time.
Related art reveals examples of optically powered sensor schemes. One has a fiber that carries optical power from the monitor or control end to the sensor end. A second fiber returns the sensor signal to the control end. Another scheme utilizes one fiber for both sending optical power to the sensor end and returning the sensor signal to the control end. There are two variations of the latter scheme. One uses a half-silvered mirror at each end. Half-silvered mirrors ae not very efficient. A second uses a dichroic mirror, having greater efficiency and whose reflectively depends on wavelength, at each end and two different wavelengths are used for the power transmission and for sensor signal return, respectively. Both variations are designed to deal with only one sensor and their feasibility has not been clearly demonstrated.